Django 1.5 mod_wsgi/simplejson fix

There were a couple issues here. Firstly, there was a bug that only manifested itself if simplejson was installed. At some point, I need to do a cleanup effort to remove all the Django<1.3 and Python<2.5 compatibility stuff.

Secondly, there was a weird issue that only manifested itself with mod_wsgi and the line checking a subclass which was not needed in my opinion. I think if the user puts an invalid JSONEncoder, then it'll throw errors which is fine.

This PR resolves #14 and #15.

Credits to @ngaranko, @tsabi and everyone else who tested and commented.